ARM has helped found Linaro (linaro.org) ~10 years ago to fix the problem with mainline Linux integration for all the participating SoC vendors.

You should expect the following manufacturers to have good mainline support:

https://www.linaro.org/membership/

I am sure your mileage will vary between vendors (I am pretty sure Qualcomm was NOT one of the founding members, but it's there now), but this at least signals an investment of money and resources.